Noise analysis of continuous GPS coordinate time series for CMONOC

The Crustal Movement Observation Network of China (CMONOC) is one of the major scientific infrastructures, mainly using Global Positioning System (GPS) measurements, to monitor crustal deformation in the Chinese mainland. In this paper, decade-long coordinate time series of 26 continuous GPS sites of CMONOC are analyzed for their noise content using maximum likelihood estimation (MLE). We study the noise properties of continuous GPS time series of CMONOC for the unfiltered, filtered solutions and also the common mode signals in terms of power law plus white noise model. In the spatial filtering, we remove for every time series a common mode error that was estimated from a modified stacking of position residuals from other sites within ∼1000 km of the selected site. We find that the common mode signal in our network has a combination of spatially correlated flicker noise and a common white noise with large spatial extent. We demonstrate that for the unfiltered solutions of CMONOC continuous GPS sites the main colored noise is a flicker process, with a mean spectral index of ∼1. For the filtered solutions, the main colored noise is a general power law process, indicating that a major number of the filtered regional solutions have a combination of noise sources or local effects. The velocity uncertainties from CMONOC continuous GPS coordinate time series may be underestimated by factors of 8–16 if a pure white noise model is assumed. In addition, using a white plus flicker noise model, the median values of velocity errors for the unfiltered solutions are 0.16 (north), 0.17 (east) and 0.58 (vertical) mm/yr, and the median values for the filtered solutions are 0.09 (north), 0.10 (east) and 0.40 (vertical) mm/yr.     

A comprehensive study of the 2016 Mw 6.0 Italy earthquake based on high-rate (10 Hz) GPS data

The 2016 M_w 6.0 Italy earthquake is successfully recorded by the near-field 10?Hz GPS and 200?Hz Strong Motion (SM) stations, providing valuable data for this study. A comprehensive study of this earthquake is carried out based on GPS data, which contains coseismic deformations analysis, noise analysis, seismic wave picking, and magnitude determination. The noise of most GPS-derived displacement waveforms can be described as a combination of white noise, flicker noise, and random walk noise after the earthquake occurrence, and the spectral indices vary significantly for most stations, implying that the seismic signals have affected the noise characteristic of GPS-derived displacement waveforms. S-transform is employed to assess the GPS capability to detect the seismic arrival time. The SM station AMT and the GPS station AMAT are in good agreement in seismic wave picking, and the difference is only 1.2?s in the north component, suggesting that the outcome of seismic wave picking using GPS data is reliable. Then, a classic empirical formula is employed to determine the moment magnitude. A robust moment magnitude (Mw 5.90) can be estimated by the nine GPS stations with about 23.9?s. If four GPS stations near the epicenter is chosen to determine the magnitude, it only take 13.0?s to retrieve a reliable preliminary (Mw 5.82) magnitude, which is 5.4?s ahead of nine stations. In addition, Cross Wavelet Transform (XWT) is adopted to measuring the correlation and phase relationship between GPS and SM records. The result of XWT analysis indicates 10?Hz GPS is capable of capturing reliable and accurate coseismic dynamic deformations, as evidenced by the XWT-based semblance being close to 1 between GPS and SM records. The above results confirm the capability of 10?Hz GPS to capture coseismic dynamic deformations, detect seismic arrival time, and determine earthquake magnitude. Moreover, rapid magnitude determination based on 10?Hz GPS data can be regarded as an important supplement to Earthquake Early Warning (EEW).     

Seismic deformation of the Mw 8.0 Wenchuan earthquake from high-rate GPS observations

High-rate GPS positioning has been recognized as a powerful tool in estimating epoch-wise station displacement which is particularly useful for seismology. In this study, station displacements during the 12 May 2008 M_w 8.0 Wenchuan earthquake are derived from the 1-Hz GPS data collected at a set of stations in China. The impacts of integer ambiguity resolution and station environment-dependent effects are investigated in order to yield more accurate results. The position accuracy of horizontal components of better than 1 cm suggests that GPS can sense the rapid position oscillation of about 2 cm in amplitude. Temporal and spatial analysis is applied to the surface displacement at station XANY and the characteristics of the movements due to Rayleigh and Love waves are detected and discussed. The comparison of GPS-derived displacement with relevant synthetic data computed based on a recently published rapture model shows a reasonable agreement in waveform. The various differences in amplitude need further investigation and also imply that rapture inversion might be improved if GPS-derived displacement is assimilated.     

Medium-scale traveling ionospheric disturbances (MSTID) modeling using a dense German GPS network

The MSTIDs are wave-like perturbations of the ionospheric plasma, which cause the most common ionospheric disturbances in mid-latitude regions. Generally the MSTIDs have velocities of several hundred meters per second and wavelengths of several hundred kilometers. The wave-like effect of the MSTID is one of the main obstacles for accurate interpolation of ionospheric corrections in a medium-scale reference GPS network. In this paper we show a new method of detecting and modeling MSTIDs using dense German GPS network. The between-epoch single difference ionospheric delays from a medium scale dense GPS network are used to estimate the parameter of the MSTID e.g. amplitude, wavelength and velocity. The efficiency of the approach is tested with data from about 320 GPS stations in and near Germany. A MSTID wave moving from east to west across Germany was observed at September 27 in 2009. Its wavelength is about 302 km, with a period of ∼7 min and velocity of about 700 m/s.     

GPS基频率标准的计量校准

GPS基频率标准具有高准确度、零漂移的特点,可广泛应用于移动通信、数字广播、计量校准等领域。介绍了GPS基频率标准的工作原理、性能描述,并进一步探讨给出了GPS基频率标准类设备的不确定度因素以及计量校准方法。     

一阶扩展差分方法在测站GPS时间误差修正中的应用

分析了全球定位系统（简称GPS）时间测量原理、时间测定误差和差分误差消除模型,给出了在航天测控站两套GPS授时系统应用一阶差分方法,即站间单差定位方法,减小误差,提高时间准确度的思路方法,并提出了多站站间单差定位的一阶扩展差分模型。该模型可以用来获取测站多套GPS授时系统的精确时间和任意2个系统时钟同步误差比对。     

基于GPS的广州地区电离层不规则体纬向漂移速度计算分析

利用广州站组建的两台短间距GPS电离层闪烁监测仪的观测数据, 分别对GPS卫星信号强度用功率谱和短间距台链互相关性两种方法计算了3次闪烁事件电离层不规则体的漂移速度. 分析结果表明, 同一不规则体会引起两台站闪烁事件的同时发生, 两种方法测量不规则体漂移速度通常在50～160m/s之间, 平均大小均在120m/s左右, 且纬向漂移速度在闪烁初期起伏较明显, 速度随闪烁时间有下降的趋势, 夜间纬向漂移方向由西向东, 广州地区漂移速度特性符合低纬其他地区不规则体漂移速度特征, 两种计算方法合理有效.      

Effects on noise properties of GPS time series caused by higher-order ionospheric corrections

Higher-order ionospheric (HOI) effects are one of the principal technique-specific error sources in precise global positioning system (GPS) analysis. These effects also influence the non-linear characteristics of GPS coordinate time series. In this paper, we investigate these effects on coordinate time series in terms of seasonal variations and noise amplitudes. Both power spectral techniques and maximum likelihood estimators (MLE) are used to evaluate these effects quantitatively and qualitatively. Our results show an overall improvement for the analysis of global sites if HOI effects are considered. We note that the noise spectral index that is used for the determination of the optimal noise models in our analysis ranged between −1 and 0 both with and without HOI corrections, implying that the coloured noise cannot be removed by these corrections. However, the corrections were found to have improved noise properties for global sites. After the corrections were applied, the noise amplitudes at most sites decreased, among which the white noise amplitudes decreased remarkably. The white noise amplitudes of up to 81.8% of the selected sites decreased in the up component, and the flicker noise of 67.5% of the sites decreased in the north component. Stacked periodogram results show that, no matter whether the HOI effects are considered or not, a common fundamental period of 1.04 cycles per year (cpy), together with the expected annual and semi-annual signals, can explain all peaks of the north and up components well. For the east component, however, reasonable results can be obtained only based on HOI corrections. HOI corrections are useful for better detecting the periodic signals in GPS coordinate time series. Moreover, the corrections contributed partly to the seasonal variations of the selected sites, especially for the up component. Statistically, HOI corrections reduced more than 50% and more than 65% of the annual and semi-annual amplitudes respectively at the selected sites.     

Estimation and analysis of GPS receiver differential code biases using KGN in Korean Peninsula

The total electron content (TEC) estimation by the Global Positioning System (GPS) can be seriously affected by the differential code biases (DCB), referred to as inter-frequency biases (IFB), of the satellite and receiver so that an accuracy of GPS–TEC value is dependent on the error of DCBs estimation. In this paper, we proposed the singular value decomposition (SVD) method to estimate the DCB of GPS satellites and receivers using the Korean GPS network (KGN) in South Korea. The receiver DCBs of about 49 GPS reference stations in KGN were determined for the accurate estimation of the regional ionospheric TEC. They obtained from the daily solution have large biases ranging from +5 to +27 ns for geomagnetic quiet days. The receiver DCB of SUWN reference station was compared with the estimates of IGS and JPL global ionosphere map (GIM). The results have shown comparatively good agreement at the level within 0.2 ns. After correction of receiver DCBs and knowing the satellite DCBs, the comparison between the behavior of the estimated TEC and that of GIMs was performed for consecutive three days. We showed that there is a good agreement between KASI model and GIMs.     

2006年12月13日太阳射电暴对GPS观测的影响

日地空间环境不仅影响航天器运行和安全, 也是导航、定位和通信等无线 电应用系统主要的误差源. 其中来自太阳L波段的射电暴被认为是全球导航卫星 系统(GNSS)稳定和性能的潜在威胁因素, 当L波段射电爆发达到一定阈值时, 将给用户带来不同程度的射电噪声干扰, 严重时会引起接收机失锁和定位服务 中断. 本文对2006年12月13日太阳射电暴对GPS造成的影响进行了研究, 利用太阳射电 观测数据、L波段闪烁观测数据和向阳面不同区域的GPS观测网数据, 分析 GPS观测对射电暴的响应. 结果表明, 此次事件对GPS观测产生了明显的影响, 射 电暴期间GPS发生幅度闪烁事件和明显失锁现象, 多个台站上空的多颗GPS 卫星 信号完全中断长达6min左右, 且多个台站上空锁定的卫星数目小于4颗, 使 得GPS定位完全失效. 相对而言, 射电暴期间日下点附近的GPS台站受到的影响 比远离日下点的大.      

海南地区电离层不规则体纬向漂移速度的观测和研究

根据中国海南富克(19.3°N,109.1°E)三点GPS观测系统2007年3月至11月的观测数据,利用互相关方法分析了三站闪烁信号的时间延迟,得出了不规则体纬向漂移的基本特征.在中国海南地区,闪烁主要发生在春秋季节,夜间不规则体的纬向漂移速度以东向为主,大小在50～150 m/s之间;平均东向漂移速度随时间呈下降趋势.另外,在闪烁刚发生时,不规则体纬向速度起伏较大,这可能与不规则体的随机起伏以及等离子体泡产生时垂直速度较大有关.中国海南地区不规则体纬向漂移速度的这些基本特征与低纬其他地区的测量结果较为一致.     

GNSS geodetic techniques for time and frequency transfer applications

We performed an initial analysis of the pseudorange data of the GIOVE-B satellite, one of the two experimental Galileo satellites currently in operation, for time transfer.¹ For this specific aim, software was developed to process the GIOVE-B raw pseudoranges and broadcast navigation messages collected by the Galileo Experimental Sensor Stations (GESS) tracking network, yielding station clock phase errors with respect to the Experimental Galileo System Time (EGST). The software also allows processing the Global Positioning System (GPS) P1 and P2 pseudorange data with broadcast navigation message collected at the same stations to obtain the station clock phase errors with respect to the GPS system time (GPST). Differencing these solutions between stations provides two independent means of GNSS time transfer. We compared these time transfer results with Precise Point Positioning (PPP) method applied to GPS data in combined carrier-phase and pseudorange mode as well as in pseudorange-only mode to show their relative merits. The PPP solutions in combined carrier-phase and pseudorange mode showed the least instability of the methods tested herein at all scales, at few parts in 10¹⁵ at 1 day for the stations processed, following a tau^−½ interval dependency. Conversely, the PPP solutions in pseudorange-only mode are an order of magnitude worst (few parts in 10¹⁴ at 1 day for the stations processed) following a tau⁻¹ power-law, but slightly better than the single-satellite raw GPS time transfer solutions obtained using the developed software, since the PPP least-squares solution effectively averages the pseudorange noise. The pseudorange noise levels estimated from PPP pseudorange residuals and from clock solution comparisons are largely consistent, providing a validation of our software operation. The raw GIOVE-B time transfer, as implemented in this work, proves to be slightly better than single-satellite raw GPS satellite time transfer, at least in the medium term. However, one of the processed stations shows a combined GPS P1 and P2 pseudorange noise level at 2 m, a factor 2 worst than usually seen for geodetic receivers, so the GPS time transfer results may not be at their best for the cases processed. Over the short term, the GPS single-satellite time transfer instability outperforms the GIOVE-B by an order of magnitude at 1 s interval, which would be due to the different characteristics of the tracking loop filters for GPS P1 and P2 on one hand and the GIOVE-B signals on the other. Even at this preliminary stage and using an experimental satellite system, results show that the GIOVE-B (and hence Galileo) signals offer interesting perspectives for high precision time transfer between metrological laboratories.     

GPS phase difference variation statistics: A comparison between phase scintillation index and proxy indices

Scintillated GPS phase observations are traditionally characterized by the phase scintillation index, derived from specialized GPS receivers usually tracking at 50 Hz. Geodetic quality GPS receivers, on the other hand, are normally tracking at frequencies up to 1 Hz. However, availability of continuously operating geodetic receivers both in time and geographical location are superior to scintillation receiver’s coverage in many parts of the world. This motivates scintillation studies using regional and global geodetic GPS networks. Previous studies have shown the usefulness of GPS estimated total electron content variations for detecting ionospheric irregularities. In this paper, collocated geodetic and scintillation receivers are employed to compare proxy indices derived from geodetic receivers with the phase scintillation index during quiet and moderately disturbed ionospheric conditions. Sensitivity of the phase scintillation indices at high latitude stations to geomagnetic activity is discussed. Global mapping of ionospheric disturbances using proxy indices from real-time 1 Hz GPS stations are also presented.     

Study of seasonal and long-term vertical deformation in Nepal based on GPS and GRACE observations

Lithospheric deformation signal can be detected by combining data from continuous global positioning system (CGPS) and satellite observations from the Gravity Recovery and Climate Experiment (GRACE). In this paper, we use 2.5- to 19-year-long time series from 35 CGPS stations to estimate vertical deformation rates in Nepal, which is located in the southern side of the Himalaya. GPS results were compared with GRACE observations. Principal component analysis was conducted to decompose the time series into three-dimensional principal components (PCs) and spatial eigenvectors. The top three high-order PCs were calculated to correct common mode errors. Both GPS and GRACE observations showed significant seasonal variations. The observed seasonal GPS vertical variations are in good agreement with those from the GRACE-derived results, particularly for changes in surface pressure, non-tidal oceanic mass loading, and hydrologic loading. The GPS-observed rates of vertical deformation obtained for the region suggest both tectonic impact and mass decrease. The rates of vertical crustal deformation were estimated by removing the GRACE-derived hydrological vertical rates from the GPS measurements. Most of the sites located in the southern part of the Main Himalayan Thrust subsided, whereas the northern part mostly showed an uplift. These results may contribute to the understanding of secular vertical crustal deformation in Nepal.     

精密GPS卫星钟差的改正和应用

分析了GPS卫生钟差的变化特性，探讨了利用GPS地面跟踪站的观测数据估算GPS卫星钟差的可行性，建立了相应的算法和软件系统，并把由地面跟踪站的实测数据估算的卫星钟差用于星载GPS定轨计算，得到优于1m的定轨精度。     

通道失配和天线互耦对GPS抗干扰天线的影响

通道失配和天线互耦对GPS(Global Positioning System)自适应抗干扰天线性能的影响,是细化GPS系统指标的基础,常被用来确定其技术实现方案.根据GPS的信号特点,利用加权矢量和干扰空间之间的正交性,分析了高频通道幅相偏差和幅相扰动、I/Q(In-phase/Quadrature)通道的幅位偏差以及天线互耦对ICR(Interference Cancellation Ratio)和SINR(Signal Interference Noise Ratio)的影响.从理论上证明了ICR主要取决于高频通道的幅相扰动和I/Q通道的幅相偏差,而与高频通道的幅相偏差和天线互耦无关,修正偏差不一定能够提高SINR,修正互耦得到的SINR的改善仅正比于天线阵S矩阵(scattering matxix)的最大奇异值.仿真实验结果验证了上述结论.     

一种新的GPS星座选择算法

在分析星座高度角和方位角对GPS定位误差影响的基础上,提出了一种新的最佳选星算法。理论分析和仿真结果表明：采用该方法选星作导航定位计算的误差和速度均优于以搜索4颗卫星组合的最小几何误差因子为计算准则的常规选星方法,且具有简单实用、工程易实现等优点,具有较强的适应性。     

Ionospheric scintillations at Guilin detected by GPS ground-based and radio occultation observations

The occurrence of ionospheric scintillations with S4 ? 0.2 was studied using GPS measurements at Guilin, China (25.29°N, 110.33°E; geomagnetic: 15.04°N, 181.98°E), a station located near the northern crest of the equatorial anomaly. The results are presented for data collected from January 2009 to March 2010. The results show that nighttime amplitude scintillations only took place in February and March of the considered years, while daytime amplitude scintillations occurred in August and December of 2009. Nighttime amplitude scintillations, observed in the south of Guilin, always occurred with phase scintillations, TEC (Total Electron Content) depletions, and ROT (Rate Of change of TEC) fluctuations. However, TEC depletions and ROT fluctuations were weak during daytime amplitude scintillations, and daytime amplitude scintillations always took place simultaneously for most of the GPS satellites which appeared over Guilin in different azimuth directions. Ground-based GPS scintillation/TEC observations recorded at Guilin and signal-to-noise-ratio (SNR) measurements obtained from GPS-COSMIC radio occultation indicate that nighttime and daytime scintillations are very likely caused by ionospheric F region irregularities and sporadic E, respectively. Moreover, strong daytime amplitude scintillations may be associated with the plasma density enhancements in ionospheric E region caused by the Perseid and Geminid meteor shower activities.     

An Integer Precise Point Positioning technique for sea surface observations using a GPS buoy

GPS data dedicated to sea surface observation are usually processed using differential techniques. Unfortunately, the precision of resulting kinematic positions is baseline-length dependent. So, high precision sea surface observations using differential GPS techniques are limited to coasts, lakes, and rivers. Recent improvements in GPS satellite products (orbits, clocks, and phase biases) make phase ambiguity fixing at the zero difference level achievable and opens up the observation of the sea surface without geographical constraints. This paper recalls the concept of the Integer Precise Point Positioning technique and discusses the precision of GPS buoy positioning. A sequential version of the GINS software has been implemented to achieve single epoch GPS positioning. We used 1 Hz data from a two week GPS campaign conducted in the Kerguelen Islands. A GPS buoy has been moored close to a radar gauge and 90 m away from a permanent GPS station. This infrastructure offers the opportunity to compare both kinematic Integer Precise Point Positioning and classical differential GPS positioning techniques to in situ radar gauge data. We found that Precise Point Positioning results are not significantly biased with respect to radar gauge data and that horizontal time series are consistent with differential processing at the sub-centimetre precision level. Nevertheless, standard deviations of height time series with respect to radar gauge data are typically [4–5] cm. The dominant driver for noise at this level is attributed to errors in tropospheric estimates which propagate into position solutions.     

Uncertainties and effects on geocenter motion estimates from global GPS observations

Global positioning system (GPS) observations can be used to estimate the geocenter motion, but are subjected to large uncertainties and effects due to uneven distribution of GPS stations and high-degree aliasing errors. In this paper, uncertainties and effects on geocenter motion estimates from global GPS observations are investigated and assessed with different truncated degrees and selected GPS network distributions based on different plate motion models, including NUVEL-1A, MORVEL56 and ITRF08. Results show that the selected GPS stations have no big effects on geocenter motion estimates based on different plate motion models, while large uncertainties are found at annual and semi-annual components when using different truncated degrees. Correlations of geocenter motion estimates from selected GPS networks with GRACE and SLR are better with truncated degree 3, and higher truncated degrees will degrade geocenter estimates. Smaller RMS also shows better results with the truncated degree 3 and the NUVEL1A has the worse results because more GPS sites are eliminated. For annual signal with truncated degree 3, four GPS strategies can reduce annual amplitudes by about 29.2% in X, 5.6% in Y, and 27.9% in Z with respect to truncated degree 1. Annual phases of all GPS solutions from MORVEL56 and ITRF08 are almost close to the GRACE solution with truncated degrees from 3 to 10, while the semi-annual signals are relatively weaker for all cases.